Evidence for transform margin evolution from the Ivory Coast–Ghana continental margin

Abstract

The relative motions between lithospheric plates can be reduced to three major types: convergence, divergence and transform motion. Convergence leads to an active margin (and eventual collision); divergence results in the construction of a rifted (passive) margin; transform motion generates a transform (or sheared) margin. Although passive margins have been extensively studied^1,2, and many models have been proposed for their origin and subsequent evolution^3–5, little is known about transform margins, with the exception of a few studies of their morphologies^6,7, shallow structures or crustal sections^8,9. Here we present the main conclusions derived from a recent study of the northern Gulf of Guinea margins, particularly off the eastern Ivory Coast and Ghana, where the continental margin is one of the best-preserved examples of an extinct transform margin^10–12. Our observations support a four-stage model for transform margin evolution. Technically active transform contacts, first between normal continental crusts and then between thinned margins, induce characteristic structures such as pull-apart grabens and shear folds. The next stage, in which thermal exchange between oceanic and continental lithos-pheres controls a complex subsidence, is followed by the transition to a true intra-oceanic fracture zone.